This invention relates to an optical system which tracks a commanded line of sight and, more particularly, to an arrangement in such a system for stabilizing the system line of sight relative to inertial space over a specified range of frequencies.
U.S. Pat. No. 4,881,800, the contents of which are hereby incorporated by reference, discloses an optical tracking and image stabilization system for use in an aircraft which tracks the movement of the pilot's helmet by developing a position command signal which causes the line of sight of the optical system to follow the line of sight of the pilot. In the disclosed system, a mirror is flexibly mounted to a rigid block which in turn is fixedly mounted to the inner gimbal of a pair of gimbals. A gyroscope having a spin axis aligned with the line of sight of the optical system is mounted to the rigid block to develop signals which are utilized to control movement of the mirror relative to the rigid block so as to nullify environmental perturbations which emanate both from the gimbal system during its positioning of the rigid block and from the aircraft.
A substantial portion of the cost of such an optical system is due to the gyroscope and its associated electronics. In addition, the incorporation of a gyroscope in such a system impacts on the amount of space required for the system. In an airborne system, such as that disclosed in the above-referenced patent, the size and weight of the system is a critical factor so that elimination of the gyroscope would be advantageous. In certain respects, optical binoculars are similar to the optical system disclosed in the referenced U.S. patent. Thus, both include a mirror mounted to a base and both track a commanded line of sight, the binoculars doing so by manual positioning whereas in the referenced patent, this is accomplished electromechanically. In the latter, the command consists of two components; the helmet angular position relative to the vehicle and the vehicle angular position, thereby creating a line of sight command relative to inertial space. Thus, the manual positioning of the binoculars is analogous to the maneuvering of the aircraft. The present invention is applicable in both cases, when the relative command is present or when it is not. In addition, it would be impractical to include a gyroscope in a pair of binoculars so that elimination of the gyroscope from an image stabilization arrangement would be advantageous to this application. Further, both are subject to environmental perturbations which vibrate the line of sight and hence the image viewed by the observer. The desired frequency response of both types of systems is also similar. Thus, if one considers the ratio of change of line of sight angle to change of base angle over a frequency band, at low frequencies this ratio should be unity because the line of sight should follow the base in a one-to-one fashion. For example, in a head-steered line of sight pointing system with a helmet mounted display, the line of sight should follow the helmet, whereas in the case of binoculars, the orientation of the line of sight should follow the nominal orientation of the binoculars. However, the jittery motions that occur at higher frequencies either due to vehicle vibration in the case of the referenced patent or because the user cannot hold the binoculars steady, especially at higher magnification, are undesirable. Thus, the ratio of change of line of sight angle to change of base angle should be smaller in that region of the frequency band so that less of this undesirable base motion is transmitted to the line of sight. Therefore, in order to reduce the cost and size of an image stabilization system, as described above, it is an object of the present invention to provide such an image stabilization arrangement without the necessity for a gyroscope and its associated electronics.
In my copending U.S. patent application Ser. No. 07/802,870, filed Dec. 6, 1991, and assigned to the assignee of the present invention, now U.S. Pat. No. 5,220,456, issued Jun. 15, 1993, I disclose a gyroless arrangement for stabilizing the line of sight in an optical tracking system of the type disclosed in the referenced patent by isolating the line of sight from angular vibration induced by the gimbal system. This gyroless arrangement is useful in applications where vehicle vibration can be ignored, but is not effective where vehicle vibration is a significant factor. It is therefore a more specific object of the present invention to provide a gyroless image stabilization arrangement of the type described which is effective over the frequency band of interest to nullify the effects of all environmental perturbations.